Continuous displacement chromatographic method

ABSTRACT

Chromatographic species are continuously separated by displacement chromatography using an apparatus that comprises an inlet distribution manifold, a particulate bed, and a plurality of collector receptacles. The particulate bed is moved with respect to the inlet distributor manifold and the collector receptacles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 07/434,913,filed Nov. 8, 1989 now abandoned, which is a divisional of applicationSer. No. 07/276,627, filed Nov. 28, 1988 (now U.S. Pat. No. 4,915,843),which applications are incorporated herein in their entireties byreference.

BACKGROUND

The present invention relates to displacement chromatography anddisplacement chromatographic apparatus.

Chromatography is a method for separating components from a mixture ofchemical substances. It is particularly useful with mixtures ofcompounds whose chemical or physical properties are so nearly identicalas to make other separation techniques difficult or impractical. Thevarious components are resolved by their selective retardation as theyare transported through a bed of sorptive medium by a moving fluid. Theseparation of the components depends upon their relative affinity forthe sorbent and the moving fluid, the length of the chromatographicdevice, and the fluid flow rate.

Four separate mechanisms or processes exist for retention of samplemolecules by the stationary phase. These, in turn, give rise to fourbasic chromatographic methods, namely, liquid-liquid, liquid-solid,ion-exchange, and size-exclusion chromatography. Liquid-liquid orpartition chromatography involves a liquid stationary phase whosecomposition is different from that of the liquid moving phase. Simplemolecules distribute between the mobile and stationary liquid phases,just as in liquid-liquid extraction with a separatory funnel. Themoving- and stationary-phase liquids must be immiscible. Liquid-solid oradsorption chromatography involves high-surface-area particles, withretention of sample molecules occurring by attraction to the surface ofthe particles. In ion-exchange chromatography, the stationary phasecontains fixed ionic groups such as sulfonate (--SO₃ ⁻) along withcounter-ions of opposite charge, e.g., Na⁺. The counter-ions are alsonormally present in the mobile phase in the form of a dissociated salt,e.g., NaCl. Simple ionic species, e.g., X⁺, are retained by an ionexchange mechanism:

    X.sup.+ +--SO.sub.3.sup.- Na.sup.+ ⃡Na.sup.+ +--SO.sub.3.sup.- X.sup.+

Finally, in size-exclusion or gel permeation chromatography the columnpacking is a porous is a porous material with pores of a certain size.Molecules that are too large are excluded from all the pores, whereassmall molecules can penetrate most of the pores. Thus, very largemolecules move through the column quickly and smaller molecules areretained by the packing. Usually, separation in size-exclusionchromatography is determined strictly by molecular size.

Two additional chromatographic methods result from modification ofliquid-liquid chromatography, namely, bonded-phase chromatography andion-pair chromatography. Bonded-phase chromatography uses an organicstationary phase that is chemically bonded to the particles in place ofthe mechanically held liquid phase used in liquid-liquid chromatography.Ion-pair chromatography can be regarded as a combination ofliquid-liquid chromatography or bonded-phase chromatography andion-exchange chromatography. Ion-pair chromatography can be carried outwith either a mechanically held liquid stationary phase or a bondedphase.

Within the four basic chromatography methods, at least two entirelydifferent, distinct separation protocols exist. These have been nameddisplacement chromatography and analytical or elution chromatography. Indisplacement chromatography at least two species B and C are sorbed ontothe stationary phase or resin which was originally loaded with a thirdspecie A having less affinity for the stationary phase than either B orC. Typically, up to 50 percent of the total capacity of the resin isloaded with species B and C. A displacer reagent consisting of a specieD is then passed through the resin. Specie D has an affinity for theresin greater than either B or C and causes the mixture of B and C (1)to be displaced from the resin, and (2) to segregate into bands of equalconcentration that elute from the resin in a head-to-tail fashion.

In analytical or elution chromatography, the mixture to be separated isinitially adsorbed on the stationary phase. An eluting species or eluentcontained in the mobile, liquid phase is then passed through the resin.The eluent also has affinity for the components of the mixture which isdifferent from the affinity of the components for the resin. The eluentneed not have any affinity for the stationary resin phase. Separation isachieved by the competition of the stationary resin phase and the mobileeluent phase for the components in the mixture. The components areresolved into symmetrical Gaussian-shaped peaks that are separated onthe resin to an extent determined by the resolution, selectivity, andefficiency of the system. The resin cannot be loaded to more than 1-5%of the total resin capacity with the mixture before resolution and peaksymmetry are adversely affected.

Analytical or elution chromatography is designed to separate smallquantities of a mixture to a very high degree of separation orresolution. It is therefore best suited and most widely used insmall-scale analytical applications. It is not a method that lendsitself readily to separation of large amounts of material.

Displacement chromatography, on the other hand, is a preparativetechnique that can separated 10 to 50 times the amount of material thatcan be processed by elution chromatography. Although resolution of thecomponents is somewhat less, displacement chromatography is the methodof choice for large-scale, industrial chromatographic separations.

In its conventional implementation, chromatography is a batch process. Avolume of the mixture to be separated is introduced at the feed end of achromatographic column (i.e., a single cylindrical tube filled withresin) and the components are resolved, in time, into individual bandsalong the column length. The separated components are then recovered atthe column exit since they are eluted at different times.

Elution chromatography is applicable to many types of mixtures and hashigh resolution capabilities and high versatility, but its deficientthroughput capacity and lack of continuous operation keep it from beinga separation technique suitable for large scale, industrial operations.Attempts have been made to increase the capacity of chromatographicdevices. One attempt is the continuous annular chromatography (CAC)system based upon elution and gradient elution chromatography.

Continuous chromatographic separations can be achieved by means of theCAC system. The CAC apparatus consists of an annular particulate bedhaving adsorbent particles packed in a space between two concentriccylinders. While the column assembly is slowly rotated about its axis,an eluent solution and a feed mixture are continuously fed to theannular bed. The eluent is uniformly fed to the entire circumference,but the feed mixture is introduced at only a fixed point on thecircumference that remains stationary in space. As time progresses,helical component bands develop and separate from the feed point, withslopes dependent upon eluent velocity, rotational speed, and thedistribution coefficient of the components. These bands are fixed inspace and exit at separated, stationary exit points at the opposite endof the annular bed. As long as conditions remain constant, the angularposition of each component band from the feed point also remainsconstant. No regeneration is required in CAC systems because the feedmixture is removed from the particulate bed by the eluent withoutchanging the chemical make-up of the particulate bed. Hence, the CACsystem is a continuous, steady-state process.

In the CAC system, the length and time coordinates characteristic ofconventional chromatographic separation is replaced with the length andangle coordinates of the rotating bed. In this respect, conventionalelution and gradient elution chromatography and elution and gradientelution CAC, respectively, are completely analogous. Therefore, elutionand gradient elution CAC should be able to perform continuously anyseparation capable of being preferred by conventional batch elution andbatch gradient elution chromatography.

Since only a small portion of the resin capacity, typically less thanabout 5 percent, can be loaded with the chromatographic species to beseparated in both elution and gradient elution CAC, the CAC methodologynevertheless has a significant drawback in that it can only separaterelatively small amounts of chromatographic species per unit volume ofresin.

SUMMARY

The present invention provides a continuous chromatographic methodwherein large amounts of chromatographic species are separated per unitvolume of resin. More particularly, in the method of the instantinvention, a feed mixture stream, a displacer reagent stream, and abarrier reagent stream are simultaneously introduced proximate one endof a particulate bed. Generally, these streams are introduced proximatethe top end of the particulate bed. The particulate bed comprises aresin. The feed stream comprises a chromatographic mixture of at leasttwo species B and C to be separated. C has a greater affinity than B forthe resin. The displacer stream comprises a chromatographic specie Dwhich has a greater affinity than C for the resin. The barrier streamcomprises a chromatographic specie A having less affinity than B for theresin. The streams are moved with respect to the particulate bed so thatsubstantially all the resin is sequentially contacted with (i) the feedmixture stream, (ii) the displacer reagent stream, and (iii) the barrierreagent stream. Substantially pure B and substantially pure C areseparately collected. Up to 50 percent of the total resin capacity canbe loaded with the chromatographic mixture to be separated.

The instant invention also encompasses an apparatus for separating atleast two chromatographic species from each other in accordance with theabove-described method. The apparatus comprises a particulate bedcontaining a resin. Means are provided for simultaneously introducingthe feed mixture stream, the displacer reagent stream, and the barrierreagent stream proximate one end of the particulate bed. In addition,means are provided for moving the stream with respect to the particulatebed so that substantially all the resin is sequentially contacted with(i) the feed mixture stream, (ii) the displacer reagent stream, and(iii) the barrier reagent stream.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with reference to the followingdescription, appended claims, and accompanying drawings where:

FIG. 1 is a perspective view of a continuous chromatographic apparatusembodying features of the instant invention;

FIG. 2 is a bottom plan view of the top manifold of the apparatus ofFIG. 1;

FIG. 3 is a two-dimensional sectional view along lines 3--3 of FIG. 1showing the distribution of various reagent sectors in a continuouschromatographic separation method embodying features of the presentinvention;

FIG. 4 is a two-dimensional representation of the distribution ofvarious reagent sectors in a continuous chromatographic separationmethod embodying features of the present invention as detailed in theExample; and

FIG. 5 is a chromatogram of the continuous displacement chromatographicseparation experiment of the Example.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to (a) a continuous displacementchromatographic method and (b) an apparatus for use therein.

With reference to the Figures, the present invention as shown in FIGS.1-3 provides a continuous displacement chromatographic (CDC) apparatus10 for use in a CDC method. The apparatus 10 comprising a stationaryinlet distributor manifold 12, a rotating annular particulate bed 14,and a plurality of stationary collector receptacles 16. The stationaryinlet distributor 12 comprises a plurality of inlet tubes 18-28 and acorresponding plurality of distribution tubes 28-38. In particular, thestationary inlet distributor 12 has a feed mixture inlet tube 18, adisplacer reagent inlet tube 20, a regenerator reagent inlet tube 22, abarrier reagent inlet tube 24, a purge inlet tube 26, and an inert gasover-pressure inlet tube 28 as well as corresponding distribution tubes,namely, a feed mixture distribution tube 30, a displacer reagentdistribution tube 32, a regenerator reagent distribution tube 34, abarrier reagent distribution tube 36, a purge distribution tube 38, andan inert gas over-pressure distribution tube 40, respectively. The exitends 42, 44, 46, and 48 of the feed mixture distribution tube 30, thedisplacer reagent distribution tube 32, the regenerator reagentdistribution tube 34, and the barrier reagent distribution tube 36,respectively, each preferably terminate in a device 50, 52, 54, and 56,respectively, adapted to distribute its respective mixture or reagentover a sector having a greater area than the cross-sectional area of itsrespective distribution tube 30, 32, 34, and 36. As shown in FIG. 2,each distribution device 50, 52, 54, and 56 can terminate in anelongated slot 58. Alternatively, one or more of the distributiondevices 50, 52, 54, and 56 can terminate in a series of nozzles (notshown).

The stationary inlet distributor manifold 12 is in O-ring sealed contact59 with the rotating annular particulate bed 14. The O-ring seal 59enables axial rotation of the inlet distributor manifold 12 relative tothe annular particulate bed 14. The rotating annular particulate bed 14is defined by a cylindrical inner core 60 and a coaxial cylindricalexterior shell 62. Sandwiched between the inner core 60 and the exteriorshell 62 is a resin 64. The annular thickness between the inner core 60and the exterior shell 62 can be almost as large as the radius of theexterior shell 62. For example, the annular thickness can be such thatthe resin 64 occupies up to about 95% of the cross-sectional area of theexterior shell 62.

A chromatographically inert retaining layer 66 is also sandwichedbetween the core 60 and shell 62 in the space V (see FIG. 3) between thetop 68 of the rotating annular particulate bed 14 and the upper edge 70of the resin 64. At least a portion of each distribution device 50, 52,54, and 56 is submerged in the inert retaining layer 66. The depth ofthe chromatographically inert retaining layer 66 is sufficient toprevent substantially any disruption of the resin 64 due to theintroduction of the feed mixture or various reagents through thedistributor devices 50, 52, 54, and 56. An exemplary chromatographicallyinert retaining layer 64 is comprised of glass beads 72. Preferably, theglass beads 72 have a particle size sufficiently small so that thedistributor devices 50, 52, 54, and 56 are capable of horizontallypassing through the glass beads 72 without any significant impedance.

The top 61 of the inner core 60 is capped with an upwardly-pointing,conically-shaped cover plate 63 having a diameter substantially the sameas the outer-diameter of the inner core 60. The lower portion 74 of theparticulate bed 14 terminates in a plurality of exit tubes 76. Theannular particulate bed 14 is rotated by a coaxial shaft 78 having oneend 79 attached to a motor 80 and another end 81 attached to the lowersurface 82 of the cover plate 63.

In accordance with a method embodying features of the instant invention,a feed mixture stream, a displacer reagent stream, a regenerator reagentstream, a barrier reagent stream, and a purge reagent stream aresimultaneously fed into the apparatus 10 through the feed mixture inlettube 18, the displacer reagent inlet tube 20, the regenerator reagentinlet tube 22, the barrier reagent inlet tube 24, and the purge inlettube 26, respectively, of the stationary inlet distributor manifold 12.The feed mixture stream comprises chromatographic species B and Cwherein C has a greater affinity than B for the resin 64. The displacerreagent stream comprises a chromatographic specie D wherein D has agreater affinity than C for the resin 64. The regenerator reagent streamis capable of displacing D from the resin 64 without otherwisesubstantially effecting the resin 64. In one version of the instantinvention, the regenerator reagent stream contains a composition, e.g.,a chelating agent, that has a greater affinity for D than the affinityof D for the resin 64. In another version of the instant invention, theregenerator reagent stream has a pH capable of removing D from the resin64. The barrier reagent stream comprises a chromatographic specie Ahaving a lesser affinity than B for the resin 64.

The feed mixture stream, the displacer reagent stream, the regeneratorreagent stream, and the barrier reagent stream exit the inletdistributor manifold 12 simultaneously through the feed distributiondevice 50, the displacer distribution device 52, the regeneratordistribution device 54, and the barrier distribution device 56,respectively. These streams separately enter the chromatographicallyinert retaining layer 66 and proceed downwardly into the resin 64. Thepurge stream exits the inlet distributor manifold 12 through the purgestream distribution tube 38 and impinges upon the center of the uppersurface 86 of the conically-shaped cover plate 63. The purge stream runsdown over the upper surface 86 of the cover plate 63 substantiallyequally in all directions and enters the chromatographically inertretaining layer 66 at those points not occupied by either the feedstream or one of the reagent streams. The purge stream remainssubstantially distinct from the feed stream and each of the reagentstreams as it passes through the retaining layer 66 and enters the resin64. To prevent any disruption in the steady state nature of the CDCmethod of the instant invention, it is preferred that all streams beintroduced through the stationary inlet distributor manifold 12 atsubstantially the same flux rate (volume/time/area).

When an inert over-pressure gas is used to expedite the CDC process,pressurizing gas enters the stationary inlet distributor 12 through thepressurizing gas inlet tube 28 and exits through the pressurizing gasdistribution tube 40 located above the upper surface 86 of theconically-shaped cover plate 63.

Initially, the resin 64 is loaded with chromatographic specie A. In oneversion of the invention, chromatographic specie A is loaded onto theresin 64 prior to the resin 64 being placed in the particulate bed 14.However, in the preferred version of the invention, chromatographicspecie A is loaded onto the resin 64 by first passing only the barrierreagent stream through the barrier distribution device 56 and throughthe resin 64 until substantially 100 percent of the resin's bindingcapacity is occupied by chromatographic specie A.

As shown in FIG. 1, the particulate bed 14 is rotated counterclockwise.Accordingly, chromatographic specie A is displaced from the resin 64 bychromatographic species B and C in the feed mixture stream passingthrough the feed distribution device 50. The feed mixture stream isintroduced over a sector that generally spans at least about 5° (or atleast about 1 percent of the horizontal cross-sectional area of theparticulate bed 14). More typically, the feed mixture stream isintroduced over a sector of about 10° to about 20° (or about 3 to about6 percent of the horizontal cross-sectional area of the particulate bed14).

Generally, sufficient feed mixture stream is introduced in the resin 64so that up to about 50 percent of the total resin capacity is occupiedby chromatographic species B and C. Typically, at least about 5 percentof the total resin capacity of the resin 64 is occupied bychromatographic species B and C. Since the amount of chromatographicspecies capable of being separated is directly proportional to the totalbinding capacity of the resin 64, it is preferred that thechromatographic species to be separated occupy as much of the resin'sbinding capacity as is consistent with displacement chromatographyprinciples. For example, it is preferred that the chromatographicspecies to be separated occupy at least about 40%, and more preferablyat least about 45%, of the resin's binding capacity.

In a preferred version of the invention, it is preferred that theparticulate bed 14 be rotated about 360° with only the feed stream beingintroduced into the resin 64 through the feed distribution device 50. Toremove substantially all chromatographic species displaced by the feedmixture, e.g., displaced A, it is preferred, as shown in FIG. 3, that aportion 84 of the purge stream 85 emanating from the purge streamdistribution tube 38 be introduced into the resin 64 between the feedmixture stream 87 and the displacer reagent stream 89. The displacerreagent stream 89, which is introduced into the resin 64 through thedisplacer distribution device 52, contains chromatographic specie Dhaving a greater affinity than chromatographic species B and C for theresin 64. Accordingly, chromatographic specie D displaceschromatographic species B and C from the resin 64. In order tosubstantially, fully separate specie B from specie C, it is preferredthat displacer specie D be introduced over a sector having a horizontallength at least as long as the minimum critical batch column length(i.e., "band length"in displacement chromotography terms) necessary toresolve specie B from specie C. The band length is the length of a batchcolumn initially occupied by the feed mixture prior to introducing thedisplacer into the batch column. Typically, the displacer specie D isintroduced over a sector of at least about 120° (or at least about 33percent of the horizontal cross-sectional area of the particulate bed14). Preferably, D is introduced over a sector that spans at least about180° (or at least about 50 percent of the horizontal cross-sectionalarea of the particulate bed 14).

Due to their differing chromatographic affinities for the resin 64,chromatographic specie C displaces chromatographic specie B as theytraverse the resin 64 and form distinct, adjacent bands 88 and 90containing substantially pure B and C, respectively. Chromatographicspecies B and C substantially separately pass through their adjacent,respective exit tubes 76 and are substantially separately collected atthe lower portion 74 of the rotating annular particulate bed 14 inseparate stationary collector receptacles 16. The displacerchromatographic specie D is removed from resin 64 the regenerator stream91 emanating from a regenerator distribution device 54. The regeneratorstream is usually introduced into the particulate bed 14 over a sectorof at least about 35° (or at least about 10 percent of the horizontalcross-sectional area of the particulate bed 14). More typically, theregenerator stream is introduced over a sector of about 35° to about100° (or about 10 to about 30 percent of the horizontal cross-sectionalarea of the particulate bed 14).

The regenerator stream 91 is in turn removed from the resin 64 byanother portion 93 of the purge stream 85. Chromatographic barrierspecie A is then reloaded onto the resin 64 by the barrier reagentstream 95 emanating from the barrier distribution device 56. The barrierstream is also usually introduced into the particulate bed 14 over asector of at least about 35° (at least about 10 percent of thehorizontal cross-sectional area of the particulate bed 14). However, thebarrier stream is more typically introduced over a sector of about 35°to about 100° (or about 10 to about 30 percent of the horizontalcross-sectional area of the particulate bed 14). Any unboundchromatographic specie A is removed from the resin 64 by a furtherbranch 97 of the purge stream 85. After removing any excesschromatographic specie A, the particulate bed 14 is substantially in thesame condition as it was at the beginning of the cycle. The cycle can berepeated as many times as necessary and, therefore, the process iscontinuous. Typically, the cycle is repeated at least twice.

As evident from the above discussion and as illustrated in FIG. 3, iy ispreferred that the feed mixture stream and each reagent stream beseparated from its neighboring reagent stream or feed mixture stream bythe purge stream 85. However, satisfactory results are also obtainablewhen various neighboring streams are not separated by the purge stream85.

In one version of the invention, the barrier and regenerator reagentstreams act in the same manner and can be combined into a single reagentstream as illustrated in FIG. 4 and discussed in the Example, infra.

The sectors for the feed mixture stream, the displacer reagent stream,the regenerator reagent stream, the barrier reagent stream, and eachportion of the purge stream can be varied in accordance with theirconcentrations, flux rates, and constituents. However, in all cases, itis preferred that the displacer stream sector be as large as possible sothat the maximum amount of chromatographic species can be separated percycle.

Any displacement chromatographic method capable of being performed by abatch process can now be performed continuously by the CDC method of theinstant invention. Exemplary chromatographic methods includeliquid-liquid, liquid-solid, ion-exchange, size-exclusion, bonded-phase,and ion-pair chromatography methods. Any organic or inorganic compoundcapable of being separated by a batch displacement chromatographicmethod can be separated continuously by the CDC method of the instantinvention. As used in the specification and claims, the term "organiccompound" means any compound containing at least one carbon atom and theterm "inorganic compound" means any compound devoid of even one carbonatom. Exemplary organic compounds that can be separated continuously bythe CDC method of the instant invention include, but are not limited to,drugs, proteins, antibodies, enzymes, and organometallic compounds.Exemplary inorganic compounds that can be separated continuously by theCDC method of the instant invention include, but are not limited to,transition metals, lanthanides, actinides, and minerals.

Accordingly, the instant invention is capable of continuously separatinglarge quantities of chromatographic species per unit volume of resin 64.

EXAMPLE Overview

The separation of rare earth elements is described in the followingexample.

Materials

A stock solution containing about 150 g/l each of neodymium (Nd) andpraseodymium (Pr) was prepared using neodymium carbonate (about 96.0%Nd₂ (CO₃)₃) and praseodymium oxide (about 96.0% min Pr₆ O₁₁).

Neodymium carbonate (about 248.7 g) was dissolved in about 300 ml watercontaining about 67 ml of about 70% nitric acid. Incremental addition ofthe carbonate was carried out to prevent loss of material duringeffervescence. Praseodymium oxide (about 187.1 g) was dissolved in about100 ml of hot 70% nitric acid. Incremental addition of the oxide to thehot acid was followed by frequent dropwise addition of about 30%hydrogen peroxide to reduce Pr(4+) to the Pr(3+) oxidation state.Following complete dissolution, the Pr mixture was boiled to a viscousstate to expel excess nitric acid and rediluted by carful addition ofwater. The solution was then brought to approximately 450 ml totalvolume, with addition of about 0.5 ml of about 70% nitric acid toprevent hydroxide formation. After cooling to room temperature, the twosolutions were combined and diluted to a final volume of about 1000 ml.Nd/Pr feed solutions for displacement chromatography experiments wereprepared by dilution of the stock solution.

The displacer reagent employed was an aqueous solution (about 5 g/l) ofN-(2-hydroxyethyl)ethylenediamine-triacetic acid (HEDTA) in the ammoniumion form (which as a chelating agent complexes with i.e., modifies, Band C, such that in their complexed, i.e., modified forms, they haveless affinity for this resin). HEDTA was dissolved in water and the pHwas adjusted to about 7.5 using aqueous ammonium hydroxide.

The resin used in the annular chromatograph experiments was Dowex 50W-X8brand (about 44-74 micron) sulfonated polystyrene-divinyl benzene cationexchange resin.

All other reagents employed were of analytical grade. All solutions wereprepared using deionized water further purified with a BarnsteadNANOpure brand water purifier.

Apparatus

A 27.9 cm diameter by 60 cm overall length continuous annularchromatograph with a 12.7 mm wide annulus was employed in this study.

Flow rates for feeds and eluents were controlled by metering pumps towithin about 1%.

A Perkin-Elmer Model 354 brand UV/Vis Spectrophotometer was used foroptical determination of the concentrations of ammonium HEDTA solutionsof Nd(3+) (578 nm) and Pr(3+) (444 nm). The millivolt signals from thedetector were converted to digital signals by an A/D conversion board(Data Translation Model 2801) interfaced to an IBM-AT brand personalcomputer for graphical display and digital data storage.

Nd and Pr concentrations were also determined using a Perkin-Elmer Model2380 brand Atomic Absorption (AA) Spectrophotometer, Nd (492.4 nm) andPr (495.1 nm), and an Applied Research Laboratories Model 3520 brandSequential Inductively Coupled Plasma atomic Emission Spectrometer(ICP-AES), Nd (404.079 nm) and Pr (440.878 nm).

Methods

An Nd/Pr binary lanthanide mixture was selected to demonstrate theconcepts because Nd/Pr represented a separation of adjacent lanthanidesthat could be monitored visually (Nd(3+) is purple and Pr(3+) is green).

Sulfonated polystyrene-divinylbenzene cation exchange resin was used asthe stationary resin phase. An aqueous solution (about 5 g/l) of HEDTAin the ammonium ion form at pH about 7.5 served as the displacer D andhydrogen ion (H⁺) was used as the barrier ion A.

The annular chromatograph was loaded with resin to a depth of about 22.8cm and topped with a layer (about 10 cm) of inert glass beads (about0.18 mm diameter). The stationary inlet distributor manifold 12 deliverysystem was configured to enable the introduction of feed, displacer,barrier, and purge water to fixed locations around the top of thechromatograph.

Each distribution tube terminated in a separate series of nozzles thatpenetrated beneath the surface of the glass bead layer but not to theextent of penetrating the top of the resin. As the particulate bedrotated, the glass beads flowed around the stationary nozzles so as notto disturb the surface of the actual resin. The glass beads, however,served to prevent lateral upward diffusion and mixing of the differentreagent streams at the top of the chromatograph. This enabled theintroduction of reagents to different sectors around the top of thechromatograph in discrete, well defined regions.

The particulate bed operated in a manner to exclude all but the reagentissuing from an individual series of distribution tube nozzles from theregion directly below those nozzles. The widths of each of the differentreagent sectors could be varied by varying the widths of each individualseries of distribution nozzles. The sequence of addition of feed,displacer, barrier, and purge around the circumference of thechromatograph could also be varied.

As shown in FIG. 4, in the experiment described below, the Nd/Pr feed100 was introduced at a fixed position (0 degrees) in a 15 degree sector101. This was followed by a sector 102 (about 195 degrees) containingthe HEDTA displacer 103. (Note: some space 104 was allowed between thefeed and displacer nozzles to prevent acid displaced by the loaded feedfrom entering the ammonium HEDTA displacer sector 102). A barrier sector106 and a purge sector 108 containing about 0.1 N nitric acid 109 anddeionized water 110, respectively, made up the remainder of thecircumference, with each of these sectors 106 and 108 being about 75degrees wide.

All the resin 64 in the particulate bed was first converted to the H⁺form and then purged with water to remove excess acid before beginning arun. This was conveniently accomplished by feeding reagent through purgedistribution tube 38 centered in the head space above the chromatographthat fed radially to all portions of the chromatograph. This includedthe sectors below any distribution nozzles not concurrently in use.Deionized water was then applied to the purge distribution tube 38 forthe remainder of a separation.

With the chromatograph rotating at 25.5° /hour, a separation wasinitialized by first applying the feed mixture 100 to the particulatebed at a flow rate of about 7.2 ml/minute. This was then followed by theaddition of the displacer reagent 103 (at a flow rate of about 130ml/minute), the barrier reagent 109 (at a flow rate of about 37ml/minute), and purge water 110 through their separate distributionnozzles, in that order and at substantially the same flux rate. Thevarious flow rates were adjusted according to the particular conditionsin a given experiment. Once suitable steady-state operating conditionswere established, the separations could be sustained continuously for anindefinite period.

The progress of a continous separation could be monitored and theresulting chromatograms recorded by attaching a slip-stream to a singleexit tube 76 at the bottom of the chromatograph and analyzing the eluateas the exit tube 76 rotated the entire 360 degrees of the chromatographcircumference. Continuous, in-line detection and monitoring was, in thismanner, conveniently accomplished.

The continuous CDC separations were conducted at room temperature.

Results and Discussion

As indicated above, a 15 degree feed sector or region 101 followed by a195 degree ammonium HEDTA displacer sector 102 and two 75 degree sectors106 and 108 containing the acid regenerator 109 and water purge 110,respectively, were employed for the Nd/Pr separation underconsideration.

With the entire resin initially in the hydrogen ion form and withdeionized water flowing to the purge distribution tube 38, the Nd/Prfeed 100 was applied to the rotating particulate bed at a concentrationof about 10 g/l in each metal. In the absence of displacer reagent 103,the 3+ lanthanides adhered strongly to the resin and accumulated in anarrow band 112 at the top of the rotating resin.

This was followed by the ammonium HEDTA displacer sector 102 at aconcentration of about 5 g/l and pH about 7.5, which resulted in a fixedregion of displacer in the particulate bed into which the feed mixtureentered. Separation and development of the individual component bands ofthe Nd 114 and Pr 116, occurred in this region.

The length of the particulate bed was adjusted to allow for completeseparation to develop before material exited the chromatograph. Thislength was determined from the flow rate and concentration of feed 100applied to the resin, the flow rate and concentration of the ammoniumHEDTA displacer 103, and the extent of the reagent sectors 102 and 106.Once separation into adjacent bands 114 and 116 and a steady-statecondition was achieved, additional resin depth would provide no furtherimprovement in separation in the displacement experiment. This isreferred to as the "isotactic" point in displacement chromatography.

An acid regenerator sector 106 (about 0.1 N nitric acid) was requiredafter the ammonium HEDTA displacer sector 102 to convert the resultantammonium ion form of the resin back to the initial hydrogen ion form.Excess acid from the regenerator sector 106 was swept from the resinbefore returning to the feed sector 101 by a final deionized water purge110. This restored the resin 64 to its initial conditions and enabledcontinuous operation.

A chromatogram obtained during this experiment is shown in FIG. 5. Thecurves indicated displacement chromatographic behavior. The separatedcomponents appeared as adjacent bands of approximately equalconcentrations and areas. Displacement chromatography was, therefore,being accomplished continuously with the procedure and apparatus of theinstant invention.

Although the present invention has been described in considerable detailwith reference to several preferred versions thereof, other versions arepossible. For example, the feed mixture stream can contain more than twochromatographic species to be separated. In addition, the inletdistributor manifold can be rotated while the particulate bed is heldsubstantially stationary, and the rotation can be in either theclockwise or counterclockwise direction. Furthermore, the particulatebed need not be annular. To illustrate, the particulate bed can belinear and the various inlet tubes can be moved continuously over thetop portion of the linear particulate bed. Also, instead of having anannular slot, the distribution device can have an opening that iswedge-shaped. Therefore, the spirit and scope of the appended claimsshould not necessarily be limited to the description of the preferredversions contained herein.

What is claimed is:
 1. A method for separating at least two chromatographic species from each other, the method comprising the steps of:(a) simultaneously introducing a feed mixture stream, a displacer reagent stream, and a barrier reagent stream proximate a first end of a particulate bed, the particulate bed comprising a resin, the feed mixture stream comprising chromatographic species B and c wherein C has a greater affinity than B for the resin, the displacer stream comprising a specie D wherein D modifies B and C such that their modified forms have less affinity for the resin, and the barrier stream comprises a chromatographic specie A wherein A has a lesser affinity than B for the resin; (b) moving the streams with respect to the particulate bed so that substantially all the resin is sequentially contacted with (i) the feed mixture stream, (ii) the displacer reagent stream, and (iii) the barrier reagent stream; (c) collecting substantially pure B; and (d) collecting substantially pure C.
 2. The method of claim 1 wherein substantially all the resin is at least twice sequentially contacted with (i) the feed mixture stream, (ii) the displacer reagent stream, and (iii) the barrier reagent stream.
 3. The method of claim 1 wherein the method further comprises the step of loading the resin with A prior to step (a).
 4. The method of claim 1 wherein step (a) includes the step of simultaneously introducing a purge stream proximate the first end of the particulate bed and step (b) includes the step of moving the streams with respect to the particulate bed so that substantially all the resin is sequentially contacted with (i) the feed mixture stream, (ii) the displacer reagent stream, (iii) the barrier reagent stream, and (iv) the purge stream.
 5. The method of claim 4 wherein step (b) includes the step of moving the streams with respect to the particulate bed so that substantially all the resin is sequentially contacted with (i) the feed mixture stream, (ii) the purge stream, (iii) the displacer reagent stream, (iv) the barrier reagent stream, and (v) the purge stream.
 6. The method of claim 4 wherein the purge stream is water.
 7. The method of claim 1 wherein the streams are held substantially fixed and the particulate bed is rotated.
 8. The method of claim 1 wherein the particulate bed is held substantially fixed and the streams are simultaneously rotated.
 9. The method of claim 1 wherein step (a) includes the step of simultaneously introducing a regenerator reagent stream proximate the first end of the particulate bed and step (b) includes the step of moving the streams with respect to the particulate bed so that substantially all the resin is sequentially contacted with (i) the feed mixture stream, (ii) the displacer reagent stream, (iii) the regenerator reagent stream, and (iv) the barrier reagent stream.
 10. The method of claim 1 wherein step (a) includes the step of simultaneously introducing a regenerator reagent stream and a purge stream proximate the first end of the particulate bed and step (b) includes the step of moving the streams with respect to the particulate bed so that substantially all the resin is sequentially contacted with (i) the feed mixture stream, (ii) the displacer reagent stream, (iii) the regenerator reagent stream, (iv) the barrier reagent stream, and (v) the purge stream.
 11. The method of claim 10 wherein step (b) includes the step of moving the streams with respect to the particulate bed so that substantially all the resin is sequentially contacted with (i) the feed mixture stream, (ii) the purge stream, (iii) the displacer reagent stream, (iv) the regenerator stream, (v) the barrier reagent stream, and (vi) the purge stream.
 12. The method of claim 1 wherein B and C are selected from the group consisting of organic and inorganic compounds.
 13. The method of claim 1 wherein step (a) includes introducing the feed mixture stream proximate the first end of the particulate bed over the zone that spans about 3 to about 6 percent of the horizontal cross-sectional area of the particulate bed.
 14. The method of claim 1 wherein step (a) includes introducing the displacer reagent stream proximate the first end of the particulate bed over the zone that spans at least about 33 percent of the horizontal cross-sectional area of the particulate bed.
 15. The method of claim 1 wherein step (a) includes introducing the barrier reagent stream proximate the first end of the particulate bed over the zone that spans at least about 10 percent of the horizontal cross-sectional area of the particulate bed.
 16. The method of claim 1 wherein step (a) includes the step of loading the resin with the feed mixture stream so that B and C occupy about 5 to about 50 percent of the total resin capacity.
 17. The method of claim 1 wherein step (a) includes introducing all the streams at substantially the same flux rate.
 18. The method of claim 1 wherein step (a) includes the step of introducing the feed mixture stream, the displacer reagent stream, and the barrier reagent stream into a chromatographically inert retaining layer and then through the resin.
 19. A method for separating at least two chromatographic species from each other, the method comprising the steps of:(a) loading a particulate bed with a chromatographic specie A, the particulate bed comprising a resin; (b) simultaneously introducing a feed mixture stream, a displacer reagent stream, a barrier reagent stream, and a purge stream into a chromatographically inert retaining layer and then through the resin, the feed mixture stream comprising chromatographic species B and C wherein C has a greater affinity than B for the resin, the displacer reagent stream comprising a specie D wherein forms chelate complexes with B and C which complexes have little affinity for the resin, and the barrier reagent stream comprises a chromatographic specie A wherein A has a lesser affinity than B for the resin; (c) moving the streams with respect to the particulate bed so that substantially all the resin is at least twice sequentially contacted with (i) the feed mixture stream, (ii) the displacer reagent stream, (iii) the barrier reagent stream, and (iv) the purge stream; (d) collecting substantially pure B; and (e) collecting substantially pure C.
 20. A method for separating at least two chromatographic species from each other, the method comprising the steps of:(a) loading a particulate bed comprising a resin with a chromatographic specie A so that substantially all of the resin is loaded with A; (b) displacing at least a portion of A from the resin with a mixture of at least two chromatographic species that are to be substantially separated from each other, each of the chromatographic species to be separated having a greater affinity than A for the resin; (c) simultaneously introducing a stream of chromatographic specie A, a stream of the mixture of chromatographic species to be separated, and a stream of a chromatographic specie D proximate a top portion of the resin bed, wherein D modifies all of the chromatographic species to be separated in such a way that their modified forms have little affinity for the resin; (d) moving the stream of A, the stream of the mixture of the chromatographic species to be separated, and the stream of D with respect to the particulate bed so that substantially all the resin is sequentially contacted with (i) the species to be separated, (ii) D, and (iii) A; and (e) substantially separately collecting each chromatographic specie in the mixture. 